Substantially dry joint compound comprising calcium sulfate, polyvinyl acetate and starch



United States Patent SUBSTANTIALLY DRY JGINT COMPOUND COM- PRISING CALCIUM SULFATE, POLYVINYL ACE- TATE AND STARCH Earl Maynard and Mitchell 1-. Ptasienski, Elk Grove Village, IlL, assignors to United States Gypsum Company, Chicago, Ill., a corporation of Illinois No Drawing. Filed Aug. 13, 1963, Ser. No. 301,897

16 Claims. (Cl. 260-114) This invention relates to an adhesive composition and more particularly to a novel type of improved joint compound characterized by fast hardening under all environmental conditions.

The use of drywall construction, i.e. a nonplastered surface, has become increasingly popular over the years such that it is estimated that over fifty percent of all new residential construction is finished in this manner. The usual drywall construction consists of gypsum wallboard panels secured to support framing members. Either one or two layers of the wallboards are used. The second layer is ordinarily adhesively secured to the first layer.

The gypsum type wallboard is made by enclosing a core of a calcined gypsum water mix between two paper layers. After this core has set and dried, the board is ready for use. It ordinarily is not practical to make wallboards over four feet wide and about twelve feet in length. As a result there are a considerable number of joints between boards in a room and these joints must be reinforced and concealed for satisfactory appearance. According to one prior art practice batten strips were applied over the joints for this purpose. This was found to be unsatisfactory and the batten strips tended to accentuate the joints.

It is desired to obtain a smooth finish wall construction similar to that obtained with conventional wet plastering methods. Accordingly great effort has been made to conceal the joints between the drywall plasterboards so as to achieve an overall smooth and continuous wall. To this end cloth, metal and finally paper tapes have been cemented over the joints between the boards to reinforce the joints, and a finishing coat of cementitious material applied over the tapes to conceal the tape and to provide a smooth surface. The preferred method at present is to recess the adjoining edges of the gypsum wallboard and embed therein, using an adhesive joint compound, a spark perforated paper tape. Several finishing or feathering layers of the joint compound are applied to conceal the tape. Finally, the work is sanded to produce a smooth surface similar to a conventional wet plaster wall.

The adhesives or binding materials commonly used in these prior art joint compounds are generally based on proteins as the primary binders. Casein or certain refined soya proteins solubilized in water by alkaline materials are used etensively for this purpose. When properly blended with fillers such as ground asbestos, mica, clays and limestone and perhaps preservatives and thickening agents such as natural gums, a joint compound is obtained which is capable of binding the joint tapes in place and providing a smooth surface. However, this type of joint compound has the serious disadvantage of requiring extended periods of time to dry and the bond is not established until drying is substantially complete.

The usual procedure for obtaining a good smooth finish is to apply the joint compound in several layers or coats. The first layer is primarily for the purpose of filling the space between the wallboards and for adhering the tape to the adjoining boards. The second layer is applied over the tape to form a smooth and even surface as is possible so as to give a continuous surface coplanar with the board. However, due to shrinkage of the joint compound upon drying and irregularities due to trowelling, etc. a third or finishing layer is generally applied. This is applied after the second coat has been sanded to a smooth layer.

Owing to the necessity of each layer being substantially dry before the subsequent one can be applied, it is obvious that considerable periods of time are required to achieve the ultimate smooth drywall. Under certain environmental conditions, such as cold, damp weather, days or weeks may be required before the Wall is completed. Such delays slow up the construction of the building and its eventual sale and occupancy.

Moreover, because of the slow drying of the known adhesive joint compounds shrinkage problems are magnified contributing further to the difiiculty of achieving a smooth wall surface. Shrinkage of the prior art joint compounds upon drying is especially serious when a second coat is applied over an earlier coat which is not completely dry. The areas of the earlier coating which were not thonoughly dry at the time of application of the second coat will subsequently shrink at some later time, possibly after the wall is finished and decorated. If this delayed shrinkage is excessive, it will necessitate a return to the job for refinishing and possibly redecorating.

From the foregoing it is apparent that if an adhesive joint compound could be made to harden quickly, not only would there be a substantial decrease in the time required for gypsum drywall construction but also there would be less shrinkage, especially that of the delayed type associated with the use of joint compound. This would indeed represent a significant advance in the art and would greatly reduce the expense and time required for refinishing or redecorating the surfaces.

It is therefore an object of this invention to provide a novel dry adhesive joint composition which after mixing with Water is particularly adapted for use in securing joint tape to wallboard and which reduces the time required for pnoper application of the tape to wallboard.

It is a further object of this invention to provide a water-dispersible dry adhesive joint composition which can be used for the finishing of joints between wallboards and which when so used exhibits comparatively low shrinkage.

It is a still further object of this invention to provide a dry adhesive joint composition which when mixed with water can be applied in a plurality of layers to joints between wallboards without lapse of long intervals between successive applications and which, when so used, minimizes delayed shrinkage.

It is also an object of this invention to provide a dry water-dispersible adhesive joint composition which after mixing with water and application to joints between wallboards will harden sufiiciently in a relatively short time to permit the application of subsequent coats in either hot, cold, dry or damp weather without requiring long intervals between successive applications.

It is a still further object of this invention to provide a process for finishing wallboard joints which will enable various successive layers of a joint finishing composition to be applied over a relatively short period of time.

In accordance with this invention there are provided novel dry adhesive compositions particularly adapted for securing joint tapes to gypsum wallboards. Several components of the novel joint composition are believed to function as binding materials with the result that the bond formed by the adhesive composition is quickly established and the time required between the application of successive coats of the joint composition is substantially reduced. The principal binding or adhesive. components of the joint composition of this invention are calcined gypsum (calcium sulfate hemihydrate) and dry powdered polyvinyl acetate. The presence of these two components in the joint composition along with the additional components required for an eminently satisfactory adhesive joint composition affords several unique advantages, particularly with respect to the relatively fast drying times and low shrinkage properties of the composition. While not wishing to be bound by any theoretical explanation of the unique results achieved, it is believed, at the present time, that when the novel adhesive joint composition is mixed with water and used in regular manner to fill and reinforce joints and to bind suitable joint tapes to gypsum wallboards, an adhesive or bonding action is exerted by both the calcined gypsum and polyvinyl acetate components of the composition. Thus, if due to environmental or other conditions drying of the joint compound tends to be slow, the calcined gypsum sets, forming a bond between the joint composition and the surfaces on which it is applied even though water is still present and the composition is not considered dry. At this time the wall surfaces can be sanded and, if desired, another layer of the adhesive joint compound applied thereto without danger of excessive shrinkage, commonly referred to as delayed shrinkage. On the other hand, if the environmental conditions are such that there is fast drying of the joint compound after application with the result that insufiicient water is present when the calcined gypsum is to set, the polyvinyl acetate component takes over and exerts a significant adhesive action and upon drying forms the desired bond. In this instance the calcined gypsum acts more in the way of a filler. Due to this dual adhesive or bonding action it is possible to apply successive coats of the adhesive joint composition of this invention with the'lapse of only several hours between successive applications. It is thus possible, depending upon circumstances, to complete a drywall construction job within a relatively short period such as, for example, a day or two as compared to several days or weeks normally required using conventional joint compositions.

The adhesive joint compounds of this invention comprise the following components, the parts being by weight:

Min. Max. Preferred Calcium sulfate hemihydrate, preferably of a normal consistency of 45 or ss 45% 90% 72% Finely divided polyvinyl acetate 4 10 7 llasticizer 0. 2 1. 0. A Set Control Additives for Caleined Gypsum:

Retarder such as commercial keiutinaccous (normal strength). 0. 1 0. S 07 4 Citric acid 0. 05 0. 2 0. l5 Accelerator such as K230i... 0.1 0. 6 0.3 Additives such as preservatives,

iiuidizing agents, thickening agents, defoameis, wetting agents, etc., approx 0.5 2.0 1.0 Fillers such as ground asbestos and mica with or without clays and/or powder limestone (1.5. to make 10 18.45 Hydrolyzed pregelled starch In lieuof part of polyvinyl acetate The calcined gypsum or calcium sulfate hemihydrate which is employed in the adhesive joint compositions of this invention is preferably the alpha type of calcium sulfate hemihydrate or alpha-gypsum. As is known in the art, alpha-gypsum is best prepared by calcining in a steam or super heated water environment as taught in U.S. Patent No. 1,901,051 to Wilbur S. Randel and Manvel C. Dailey. Ordinary or beta-gypsum which is generally made by calcining in a rotary kettle or calciner can also be used but is less preferred than alpha-gypsum. The consistency of the calcined gypsum component should be about 45 cc. or less. By consistency is meant the amount of water, expressed in cubic centimeters per 100 grams of plaster, which is required to form a patty about 3%" in diameter when fifty grams of the plaster is stirred for seconds with a spatula using 50 to 60 complete revolutions and poured from a height of between 1 to 1 /2 inches. The principal advantages of using a low consistency gypsum component is that smaller amounts of water are needed to reach a fluid condition and hence drying out of the joint composition after application is facilitated. The amount of gypsum to employ in the joint compositions of the invention can vary from about 45 to 90% by weight, and preferably is about by weight.

The polyvinyl acetate component is employed in the joint compositions of the invention as a dry powder and can be obtained by spray drying a polyvinyl acetate emulsion. The dry polyvinyl acetate powder can be ob tained, for example, by the process described in U.S. Patent No. 2,800,463 to Morrison and the various polyvinyl acetate powders which are commercially available can be used satisfactorily. The polyvinyl acetate component is employed in the joint compositions of the invention in an amount ranging from about 4 to about 10% by weight. It is preferred to use polyvinyl acetate which is stable to boron and boron compounds and it is important that the dry polyvinyl acetate be dispersible or emulsifiable in water and exert an adhesive action when the water is removed.

A plasticizer for the polyvinyl acetate component is included in the joint compositions of this invention. The plasticizer is one which is suitable for use in joint compositions of this type of since the compositions exist in dry form, a dry or solid type of plasticizer is required. One preferred plasticizer comprises a mixture of ortho and paratoluene sulfonamides such as that sold commercially under the tradename Santicizer 9 (25% ortho% para). The amount of plasticizer can vary between wide limits, although larger amounts such as 20% or more by weight of the dry polyvinyl acetate do not appear to be necessary or particularly advantageous. Generally, the plasticizer is employed in lesser amounts such as 5% or preferably 10% by weight of the polyvinyl acetate. The particle size of the dry plasticizer as well as the other materials used in the compositions of the invention should be such as will pass a 60 mesh sieve U.S. series.

In order that the adhesive joint compositions be suitable for general use under varying use conditions, it is desired to control the setting time of the calcined gypsum component of the compositions. This is done, as is known in the art, by the use of suitable set control additives which include set retarders and set accelerators. While the set of calcined gypsum can be controlled over varying periods such as from about 5 minutes to 24 hours or more, it is found that it is preferable in the case of the present joint compounds to control this setting to several hours at 75-80 F. Various known retarders and accelerators can be employed in the compositions of the invention to achieve these setting times. These known additives includes set accelerators such as aluminum sulfate, potassium sulafte, terra alba, or set retarders such as the commercial keratinaceous retarders, sodium citrate, citric acid and the like. The amount of the set control additives can be varied as it necessary to provide setting times for the calcined gypsum of from 1 to 5 'hours. It has been found that the presence of citric acid in the joint compositions aifords better control over the setting time and with more uniform results. The use of citric acid either by itself or combined with other set control additives such as, for example, a commercial keratinaceous retarder, constitutes a preferred embodiment of the invention.

As shown in the table above, various components in addition to the lpolyvinyl acetate and gypsum binders can be employed to provide eminently satisfactory joint compositions having widespread utility. For example, such filler materials as ground mica and ground asbestos are usually preferred to improve crack resistance and trowelability. In general, any standard filler material can be selected to provide optimum balance in such properties as ease of mixing, proper consistency, application qualities and the like. Additives other than fillers can also be incorporated in the joint compositions of the invention if desired to achieve certain desired results. Thus, for ex ample, there can be employed additives such as preservatives, fungicides, wetting and fluidizin-g agents, thickening agents such as methyl cellulose, alginates and the like. Fillers and additives as described herein are well known to the art and are generally employed in joint compounds. The term filler-additive as employed herein connotes one or more of these conventional materials.

The preferred joint compositions of the invention will have a consistency between 55 and 60 cc. for a 400:20 Brabender value at 77 F. This value is obtained on a viscometer manufactured commercially. This instrument is equipped with a 250 centimeter-gram torsion head and operates at 75-78 r.p.m. spindle speed. The setting time is determined at a temperature between 75 and 80 F. after mixing with the amount of water required to .give the above Brabender value. A Vicot needle can be utilized to determine when the gypsum has set.

In a further preferred embodiment of the invention part of the polyvinyl acetate binder is replaced with a dry partially converted pregelled starch. This type of starch, which is low in cost, functions well as a binding material and favorably influences the formation of a good bond, particularly under low temperature and low humidity drying conditions. The proportion of starch to polyvinyl acetate should not exceed about 1:1 and lower amounts of starch are generally preferred. I

A suitable starch may be prepared by suspending unmodified cornstarch (globe pearl) at 20 concentration in 0.1 N sulfuric acid and heating for six hours in a jacketed container held at 130-135 F., at the end of which time the crosses on the starch particles when viewed with a dark field microscope become indistinct but do not entirely disappear. At this point the sulfuric acid is neutralized with soda ash to pH 7 and the thus modified starch pregelatinized by cooking to 1 80 190 F. for a few minutes. The cooked starch is then flash dried in a thin film by exposure to 300 F. temperatures for a few seconds and recovered for use. Analysis of the starch so prepared shows 25.8% cold-water soluble and 7.6% moisture.

It should be appreciated that dry partially converted pregelled starches made by other methods are commercially available and are suitable for use in the present invention. Unrefined corn flour is also a suitable raw material for a satisfactory starch product. The term starch when hereinafter employed refers to a product similar to that above described.

With the use of the joint compositions of this invention it will be found that there will be a marked decrease in the overall time required to finish joints between gypsum wallboards. The entire taping and finishing of the joints in an average house have been completed within a single day. This is a marked advance in the art wherein utilizing conventional prior art joint compounds and applying three or four coats as much as several weeks were required to complete the job. With the novel joint compounds described herein it is not necessary to wait for the moisture to be removed completely before the application of a second layer or coat since the adhesive action takes place by the setting of the cementitious gypsum material. After the gypsum sets, the surface is sufficiently hard to permit sanding thereof and is receptive to the application of another layer of the joint compound. =By control of the set, hardening of the applied material can thus be tailored to take place at a specific interval after mixing with water and as a result the contractor or applicator is better able to schedule his work and is not plagued by delays due to slow drying time. With a joint compound having a 4 hour hardening or setting time an applicator with adequate labor can completely surface the tape and seal the joint nail heads in an average house in a single day.

The adhesive action minimizes the danger of of the cementitious gypsum also delayed shrinkage which some- 'the spirit of the invention times takes place when a second coat of a joint compound is applied over another which has not dried completely.

The improved compounds of this invention will form a good adhesive bond under varying atmospheric conditions; if it is cold and damp the plaster or calcined gypsum will harden and form a bond which can be designated a cementitious type bond. If it is hot and dry and the plaster does not set before the moisture is removed, adhesion is then achieved by means of the polyvinyl acetate, with or without the starch. This dual action enables the low consistency formulation to be used. Heretofore with plaster type compositions which are applied in a thin layer it is necessary either to use a fast set or a higher consistency plaster so as to prevent the removal of the water before the set has taken place. The use of small amounts of water is desirable since, as a general rule, the lesser amount of water employed the better the surface. The prior art joint compounds which are based primarily upon protein substances as a primary binder, generally require higher amounts of water.

The improved adhesive joint compounds of this invention are not only admirably suited for finishing joints but also can be used as a plaster to cover the entire surface coat of the wall, particularly in the form of a thin coat. Likewise, the novel joint compositions are ideal for nail spotting so 'as to obtain a substantially fiat wall surface.

It should be apparent to those skilled in the art that the ingredients of the joint compositions of this invention can be selected and proportioned so as to obtain optimum trowelling and general application properties such as proper stickiness, open-time, feathering ability, ease of sanding and so forth.

Those modifications and equivalents which fall within and the scope of the appended claims are to be considered part of the invention.

We claim:

1. A substantially dry composition adapted for use in finishing joints between wallboards when mixed with water comprising a mixture of calcium sulfate hemihydrate in an amount from about 45 to by weight, set control additives for said calcium sulfate hemihydrate, dry finely divided polyvinyl acetate, a dry plasticizer for said polyvinyl acetate.

2. A composition according to claim 1, wherein a portion of the polyvinyl acetate is replaced with partially converted pregelled starch, the amount of said starch being not more than the amount of polyvinyl acetate present in the composition.

3. The composition of claim 1 wherein the calcium sulfate hemihydrate is alpha type calcium sulfate hemihydrate having a consistency of not more than about 45 cubic centimeters per grams.

4. A composition according to claim 1 wherein the set control additives are selected to provide a setting time in the calcium sulfate hemihydrate of less than about 5 hours.

5. A composition according to claim 1 wherein citric acid is employed as a set control additive.

6. A composition according to claim 1 wherein the plasticizer for said polyvinyl acetate comprises a mixture of ortho and para toluene sulfonamides.

7. A substantially dry composition adapted for use in finishing joints between wallboards when mixed with water comprising from about 45 to 90% by weight of calcium sulfate hemihydrate, set control additives for said calcium sulfate hemihydrate selected to provide a setting time in the calcium sulfate hemihydrate of less than about 5 hours, from about 4 to about 10% by weight of a finely divided polyvinyl acetate, from about 0.2 to 1% by weight of a dry plasticizer for said polyvinyl acetate.

8. A composition according to claim 7, wherein a portion of the polyvinyl acetate is replaced with partially converted pregelled starch, the amount of said starch being not more than the amount of polyvinyl acetate present in the composition.

9. A substantially dry composition adapted for use in finishing joints between wallboards when mixed with water comprising about 72% by Weight of calcium sulfate hemihydrate, set control additives for said calcium sulfate hemihydrate selected to provide a setting time in the calcium sulfate hemihydrate of less than about 5 hours, about 7% by weight of dry finely divided polyvinyl acetate, about 0.7% by weight of a dry plasticizer for said polyvinyl acetate, and filler-additives.

10. A substantially dry composition adapted for use in finishing joints between wallboards when mixed with water comprising about 72% by weight of calcium sulfate hemihydrate, about 7% by weight of finely divided polyvinyl acetate, about 0.7% by weight of a plasticizer for said polyvinyl acetate, about 0.4% by weight of a set retarder, about 0.15% of citric acid and about 0.3% pctassium sulfate.

11. A substantially dry composition adapted for use in finishing joints between wallboards when mixed with water comprising a binder and finely divided filler-additives, said binder comprising a mixture of calcium sulfate hemihydrate in an amount from about 45 to 90% by weight, set control additives for said calcium sulfate hemihydrate, dry finely divided polyvinyl acetate and a dry plasticizer for said polyvinyl acetate.

12. A composition according to claim 11, wherein a portion of the polyvinyl acetate is replaced with partially converted pregelled starch, the amount of said starch being not more than the amount of polyvinyl acetate present in the composition.

13. A substantially dry composition adapted for use in finishing joints between wallboards when mixed with water comprising a binder and finely divided filler-additives, said binder comprising from about 45 to 90% by weight of calcium sulfate hemihydrate, set control additives for said calcium sulfate hemihydrate selected to provide a setting time in the calcium sulfate hemihydrate of less than about 5 hours, from about 4 to about 10% by weight of a finely divided polyvinyl acetate and from about 0.2 to 1% by weight of a dry plasticizer for said polyvinyl acetate.

14. In the process of finishing wallboard joints by applying thereto an aqueous composition adapted to adhere joint tapes thereto and to form a hard smooth surface thereover, the improvement which consists in using as the aqueous composition a mixture of calcium sulfate hemihydrate in an amount from about 45 to 90% by weight, set control additives for said calcium sulfate hemihydrate, dry finely divided polyvinyl acetate and a dry plasticizer for said polyvinyl acetate.

15. In the process of finishing wallboard joints by applying thereto an aqueous composition adapted to adhere joint tapes thereto and to form a hard smooth surface thereover, the improvement which consists in using as the aqueous composition a binder and finely divided filleradditives, said binder comprising a mixture of calcium sulfate hemihydrate in an amount from about 45 to 90% by weight, set control additives for said calcium sulfate hemihydrate, dry finely divided polyvinyl acetate and a dry plasticizer for said polyvinyl acetate.

16. A process of finishing wallboard joints with an aqueous composition which comprises forming a dry mixture of filler-additives and a binder, said binder comprising a mixture of powdered calcium sulfate hemihydrate in an amount from about 45 to 90% by weight, powdered set controlling additives for said calcium sulfate hemihydrate, finely divided polyvinyl acetate and a plasticizer for said polyvinyl acetate, mixing said dry mixture with water to application fluidity to form a thick aqueous plaster slurry and applying said slurry in a thin layer to wallboard joints.

Millan Publishers.

WILLIAM H. SHORT, Primary Examiner. J. NORRIS, Assistant Examiner. 

1. A SUBSTANTIALLY DRY COMPOSITION ADAPTED FOR USE IN FINISHING JOINTS BETWEEN WALLBOARDS WHEN MIXED WITH WATER COMPRISING A MIXTURE OF CALCIUM SULFATE HEMIHYDRATE IN AN AMOUNT FROM ABOUT 45 TO 90% BY WEIGHT, SET CONTROL ADDITIVES FOR SAID CALCIUM SULFATE HEMIHYDRATE, DRY FINELY DIVIDED POLYVINYL ACETATE, A DRY PLASTICIZER FOR SAID POLYVINYL ACETATE.
 2. A COMPOSITON ACCORDING TO CLAIM 1, WHEREIN A PORTION OF THE POLYVINYL ACETATE IS REPLACED WITH PARTIALLY CONVERTED PREGELLED STARCH, THE AMOUNT OF SAID STARCH BEING NOT MORE THAN THE AMOUNT OF POLYVINYL ACETATE PRESENT IN THE COMPOSITION. 